The anterior pituitary hormone, prolactin, has been thought to be an anti-insulin hormone. The purpose of this study is to use a model of chronic hyperprolactinemia to characterize the effects of prolactin on carbohydrate and fat metabolism. Rats receive extra anterior pituitary glands under the kidney capsule, creating a hyperprolactinemic but otherwise normal animal. The studies so far have shown that the hyperprolactinemic rats have glucose tolerance slightly better than that of control rats. The serum glucose levels in hyperprolactinemic rats after an 18- or 24-hour fast are also lower than levels in normal rats. As a means of understanding this phenomenon, fat cells from hyperprolactinemic rats have been studied. In vitro, these isolated adipocytes tend to oxidize more glucose to carbon dioxide than fat cells from control rats. Further studies are in progress to determine the mechanism of this finding.